Before the 1990s, scientists weren’t sure that planets existed around other stars as they do around the Sun. It took the discovery of two planets, Poltergeist and Phobetorare, orbiting a pulsar in 1992, and then the discovery of 51 Pegasi b around a distant Sun-like star in 1995 — for which Michel Mayor and Didier Queloz shared the Nobel Prize in Physics in 2017 — to confirm that planets indeed exist outside our solar system.
Since the mid-1990s, the catalogue of extrasolar planets, or “exoplanets”, has burgeoned to the point where it now contains over 5500 entries, an incredible amount of growth in just three decades.
Yet as more and more exoplanets are discovered and as humanity probes these worlds deeper than ever before, we are rapidly learning that the Milky Way is packed with a wealth of radically diverse planets. In fact, some of these worlds are truly “alien”, making our own solar system’s planets look reassuringly boring!
One example of this is WASP-17 b and its newly uncovered clouds of the oxygen and quartz.
WASP-17 b is a truly alien exoplanet
Located around 1320 light-years from Earth and discovered in 2009, WASP-17 b is a gas giant 1.9 times the size of Jupiter but only has 0.78 times its mass, making it one of the “puffiest” planets we’ve ever seen.
WASP-17 b orbits incredibly close to its parent star, completing an orbit in just 3.7 Earth days. This proximity means WASP-17 b is classed as a “hot Jupiter”. The planet is also so close to its star that it doesn’t revolve, making it tidally locked like the Moon is to Earth. This means one side of WASP-17 b always faces its star, and one side always points out to space.
The closeness of WASP-17 b to its star has another consequence: As it is blasted by intense radiation, the exoplanet’s “day side” surface temperature soars to an estimated 1550 Kelvin or around 1280 degrees Celsius.
This might not be as hot as some of its fellow exoplanets like WASP-76 b — another hot Jupiter which has surface temperatures as high as 2400 degrees Celsius, hot enough to melt iron which falls as rain on the planet’s cooler side — but it is still enough to give rise to some extraordinary and very alien phenomena.
The James Webb Space Telescope (JWST) has just discovered a new and extraordinary example of such a phenomenon around WASP-17 b. The most powerful space telescope ever placed into orbit used its IRI (Webb’s Mid-Infrared Instrument) to find pure silica (SiO2) particles or quartz in the atmosphere of WASP–17 b, the first time this has been observed around any exoplanet.
“We were thrilled!” University of Bristol researcher David Grant said in a NASA press release. “We knew from Hubble observations that there must be aerosols — tiny particles making up clouds or haze — in WASP-17 b’s atmosphere, but we didn’t expect them to be made of quartz.”
Grant is the first author of a paper documenting the JWST’s findings published in the Astrophysical Journal Letters.
Never seen before clouds of quartz
Silicates are minerals rich in silicon and oxygen, and are found throughout the Earth, the moon, and the solar system’s other terrestrial (rocky) worlds.
As such, it is not too surprising that silicates are also common across the Milky Way, with astronomers thus far discovering them in the atmospheres of exoplanets and brown dwarfs — bodies that form like stars but lack the mass to trigger the nuclear fusion of hydrogen, hence the nickname “failed stars”.
There is a key difference here when compared to those historic discoveries, however. Previous silicates seen in the atmosphere of exoplanets have been magnesium-rich silicates, like olivine and pyroxene, not pure quartz.
“We fully expected to see magnesium silicates,” paper co-author and University of Bristol researcher Hannah Wakeford added in the same press release. “But what we’re seeing instead is likely the building blocks of those, the tiny ‘seed’ particles needed to form the larger silicate grains we detect in cooler exoplanets and brown dwarfs.”
The team was able to make this detection thanks to the super-puffiness of WASP-17 b, which, along with its short orbital period, makes it an ideal candidate for study with transmission spectroscopy. The process measures the effect that passing through a planet’s atmosphere has on starlight, and because chemical elements absorb light at specific and unique features, it can tell scientists what makes up those atmospheres.
Investigating the atmosphere of a hot Jupiter
JWST observed WASP-17 b for around ten hours, taking over 1275 brightness measurements as the hot Jupiter crossed or “transited” the face of its star, filtering starlight through its atmosphere. This revealed an unexpected signal not consistent with clouds of magnesium silicates or other possible high-temperature aerosols like aluminum oxide. This signal only made sense in terms of quartz clouds.
These crystals of quartz likely share a pointy angular shape with Earth-based quartz, albeit in much smaller fragments no wider than around one-millionth of a centimeter across.
Also unlike quartz on Earth, the silicate around WASP-17 b does not originate from rocks, say, blown into the atmosphere from the planet’s surfaces by strong winds, but instead comes from the atmosphere itself. The high temperatures of WASP-17 b and the relatively low pressures in its higher atmosphere allow solid crystals to form directly from gas, skipping a liquid phase.
Determining the composition of the clouds of exoplanets is vital in revealing these worlds’ overall conditions and figuring out how they were formed.
“If we only consider the oxygen that is in these gases and neglect to include all of the oxygen locked up in minerals like quartz (SiO2), we will significantly underestimate the total abundance,” Wakeford continued. “These beautiful silica crystals tell us about the inventory of different materials and how they all come together to shape the environment of this planet.”
The team isn’t currently sure how much quartz lurks in the clouds of WASP-17 b, but they suspect most of it is located along the dividing line between the exoplanet’s day and night sides — called the terminator line. This also means winds of thousands of miles per hour could blow the clouds from the cool night side to its scorching hot day side, where the silicates are vaporized again.
If this can be confirmed, glassy particles whipping around at thousands of miles per hour will be another phenomenon that makes exoplanets seem completely alien and the solar system relatively safe in comparison.
Reference: D. Grant., et al., JWST-TST DREAMS: Quartz Clouds in the Atmosphere of WASP-17b, Astrophysical Journal Letters, (2023). DOI: 10.3847/2041-8213/acfc3b
Feature image: This artist’s concept shows what the hot gas giant exoplanet WASP-17 b could look like based on observations from ground- and space-based telescopes, including NASA’s Webb, Hubble, and retired Spitzer space telescopes. Credit: NASA, ESA, CSA, and R. Crawford (STScI)